Conventionally, a capacitance measurement apparatus that measures a capacitance such as a capacitor microphone whose value of electrostatic capacitance changes for various frequencies is known. FIG. 6 is a circuit diagram showing an example of a conventional capacitance measurement apparatus. As shown in FIG. 6, the conventional capacitance measurement apparatus includes an operational amplifier 112, an AC voltage generation apparatus 113, a measuring capacitance 114 and feedback resistance 116 as feedback impedance. The feedback resistance 116 and the measuring capacitance 114 are connected via a signal line 117. The signal line 117 is connected to one terminal of the operational amplifier 112. Additionally, one terminal of the AC voltage generation apparatus 113 is connected to the electrode of the measuring capacitance 114 opposite to the side connected to the signal line 117. Additionally, the other terminal of the AC voltage generation apparatus 113 is grounded.
Here, the measuring capacitance 114 changes its own electrostatic capacitance Cs in response to received physical quantity (such as acceleration, pressures, gas, light and sound wave). Additionally, the AC voltage generation apparatus 113 is to generate an operation signal Vin that is applied to the measuring capacitance at the time of measuring the capacitance.
As a capacitance measurement action of the conventional capacitance measurement apparatus shown in FIG. 6, when the operation signal (voltage: Vin) is generated from the AC voltage generation apparatus 113, the voltage Vin of the operation signal is applied to both terminals of the measuring capacitance 114. Hereby, electric current flows to the measuring capacitance 114. In this case, since input impedance of the operational amplifier 112 is ideally infinite, all the electric current that flows to the measuring capacitance 114 flows to the feedback resistance 116. Hereby, it is possible to output the output voltage Vout corresponding to electrostatic capacitance Cs from a signal output terminal 118. Then, the electrostatic capacitance Cs can be obtained by performing various kinds of signal processing based on the output voltage Vout of a detection signal.
Since the conventional capacitance measurement apparatus shown in FIG. 6 uses the feedback resistance 116 as the feedback impedance, the output voltage Vout of the signal output terminal 118 has a frequency characteristic as represented by the following equation (1).
                    Vout        =                              -            Rf                    ⁢                      {                                                                                (                                          Cd                      +                                              Δ                        ⁢                                                                                                  ⁢                                                  C                          ·                          sin                                                ⁢                                                                                                  ⁢                                                  ω                          c                                                ⁢                        t                                                              )                                    ·                                      ω                    in                                    ·                  cos                                ⁢                                                                  ⁢                                  ω                  in                                ⁢                t                            +                                                          (        1        )                                                                          ⁢                      Δ            ⁢                                                  ⁢                          C              ·                              ω                c                            ·              cos                        ⁢                                                  ⁢                          ω              c                        ⁢                          t              ·              sin                        ⁢                                                  ⁢                          ω              in                        ⁢            t                    }                ⁢        Vi                                        
Referring to the above equation (1), Vi is an amplitude of the signal Vin from the AC voltage generation apparatus 113 and ωin is angular velocity. Additionally, Cd is a standard capacitance value of the measuring capacitance 114; ΔC and ωc are a capacitance value and angular velocity of a capacitance changing part in the measuring capacitance 144, respectively. The above equation (1) includes a term in which the angular velocity ωc of the capacitance changing part is proportionate to the capacitance value ΔC of the capacitance changing part. Because of this, since the output voltage Vout is proportionate to the frequency (ωc/2 π=fc) of the capacitance changing part, the output voltage Vout has the frequency characteristic. Consequently, it is necessary to set up newly a processing circuit that does not have the frequency characteristic in a subsequent stage, and therefore there is a problem that the size of the circuit becomes large.
There, technology that the feedback impedance is configured not by the resistance but by the capacitance is proposed. FIG. 7 is a circuit diagram showing such a capacitance measurement apparatus. Referring to FIG. 7, this capacitance measurement apparatus is configured by a feedback capacitance 115. The output voltage Vout of this circuit is represented by the following equation (2).Vout={(Cd+ΔC·sin ωct)/Cf}Vin  (2)
As shown in the above equation (2), when the feedback impedance is configured by the feedback capacitance 115 (capacitance value: Cf), since the electric charge stored in the electrostatic capacitance Cs and that stored in the capacitance value Cf of the feedback capacitance 115 are equal, it is possible to maintain the amount of electric charge of the signal line 117 constantly. Consequently, the output voltage Vout does not include a term that is proportionate to the angular velocity ωc. Therefore, since the circuit output does not have dependence on the capacitance change frequency, there is no need to set up newly the processing circuit that does not have the frequency characteristic in a subsequent stage. As a result, it is possible to prevent the size of the circuit from becoming larger.
However, if the feedback impedance is configured by the feedback capacitance 115 as the technology shown in FIG. 7, since the direct current does not flow to the signal line 117 situated between the feedback capacitance 115 and the measuring capacitance 114, the signal line 117 becomes floating state electrically. For this reason, the electric potential of the signal line 117 becomes unstable and there is a disadvantage that the circuit does not operate normally such as the circuit output being saturated with the power voltage.
To prevent such a disadvantage, as shown in FIG. 7, it is conceivable to fix the electric potential of the signal line 117 by connecting resistance 119 between the signal line 117 and GND.
However, as is described above, in the case of fixing the electric potential by the resistance 119, at the time of measuring the capacitance, electric current may flow through the resistance 119. In that case, since the amount of electric charge varies, there is a problem that the sensibility of the capacitance measurement apparatus decreases. Therefore, it is difficult to execute an accurate capacitance measurement.
Additionally, when Vin is applied to the measuring capacitance 114, even though the signal line 117 is covered by a shielding wire (not illustrated) and the shielding wire and the signal line 117 are made to be at the same electric potential by imaginary short and then the shielding wire and the signal line 117 are dropped on the GND, the signal line 117 does not become the GND in the real operational amplifier 112 and a little signal of Vin appears on the signal line 117. Therefore, since parasitic capacitance is generated between the shielding wire and the signal line 117, it is difficult to execute an accurate capacitance measurement because of an effect of the parasitic capacitance.
The present invention is done to solve the above-mentioned problems and it is an object of the present invention to provide an electric potential fixing apparatus that can prevent the amount of electric charge in the connection line between the first capacitance and the second capacitance from changing.
It is another object of the present invention to provide a capacitance measurement apparatus that can execute an accurate capacitance measurement without lowering the sensibility even when the electric potential of the connection line between the first capacitance and the second capacitance is fixed.
It is yet another object of the present invention to provide an electric potential fixing method for enabling to prevent the amount of electric charge in the connection line between the first capacitance and the second capacitance from changing.